An ink jet head has a large number of piezoelectric elements, which are capacitance type actuators, as actuators for ink jet. A driving device shown in FIG. 1 for driving these actuators is mounted on the ink jet head.
In FIG. 1, a DC power supply P outputs DC voltage “E” of a fixed level. A large number of series circuits such as a series circuit of switch elements S12 and S11, a series circuit of switch elements S22 and S21, and a series circuit of switch elements Sn2 and Sn1 are connected to an output end of the DC power supply P. An actuator Z1 as a piezoelectric element is connected between an interconnection point of the switch elements S12 and S11 and an interconnection point of the switch elements S22 and S21. Actuators Z2 to Zm are respectively connected between the remaining series circuits. The actuators Z1, Z2, . . . , and Zm have capacitors C, respectively. The actuators Z1, Z2, . . . , and Zm operate according to the sequence of charging and discharging through the switch elements to jet inks.
First, when the switch elements S12 and S21 are turned on, voltage “E” in a forward direction is applied to the actuator Z1 from the DC power supply P and the actuator Z1 is charged. Specifically, as indicated by an arrow, electric charge Q flows out from the DC power supply P and is stored in the actuator Z1 via the switch elements S12 and S21. The electric charge Q is represented as Q=C·E.
During this charging, energy extracted from the DC power supply P is “Q·E”. Energy stored in the actuator Z1 is “(Q·E)/2”. A difference “(Q·E)/2” between the energy “Q·E” and the energy “(Q·E)/2” is consumed by resistance components on a charging path and changes to heat.
Subsequently, the switch elements S12 and S21 are turned off. Thereafter, when the switch elements S12 and S22 are turned on, the energy “(Q·E)/2” stored in the actuator Z1 is discharged on a path passing through the switch elements S12 and S22 from the actuator Z1 as indicated by a broken line arrow. The energy is consumed by resistance components on the discharging path and changes to heat.
A change in the voltage “E” applied to the actuator Z1 is shown in FIG. 2. T1 indicates a charging period by forward energization (application of the voltage “E” in the forward direction). T2 indicates a discharging period of energy stored by the forward energization. T3 indicates a charging period by a backward charging (application of the voltage “E” in the backward direction). T4 indicates a discharging period of energy stored by the backward charging.
In a period of the forward energization including the charging period T1 and following period just before T2, the actuator Z1 is deformed and an ink for jet is refilled in a channel. In the next the discharging period T2, the deformation of actuator Z1 is reset and the ink in the channel is jet. The following period of the charging period T3 in the backward direction and the discharging period T4 are a dumping period for jet.
In the driving device having such a configuration, the energy “Q·E” extracted from the DC power supply P during the charging is equivalent to a sum of the energy “(Q·E)/2” consumed by the resistance components on the charging path and the energy “(Q·E)/2” consumed by the resistance components on the discharging path for each forward and backward charging and discharging.
When the actuator Z1 as the capacitance type actuator is driven, only a very small part of the energy extracted from the DC power supply P is actually used for the operation of the actuator Z1. Most of the energy is consumed by the resistance components on the charging and discharging paths.
The resistance components on the charging and discharging paths are present not only in the actuator Z1 but also on circuits. The resistance components in the actuator Z1 are so-called equivalent series resistance of the actuator Z1. If the equivalent series resistance of the actuator Z1 is smaller, heat generation of the actuator Z1 decreases but heat generation of the circuits increases. Conversely, if the resistance components of the circuits are smaller, heat generation of the circuits decreases but heat generation of the actuator Z1 increases. A total of a heat value of the actuator Z1 and a heat value of the circuits is fixed. Energy consumed by the resistance components on the charging and discharging paths is also fixed.
In general, as means for reducing a heat value of an actuator, there is known a technique for applying voltage of a trapezoidal shape or a triangular shape on the actuator to prevent applied voltage to the actuator from rapidly rising. However, in this case, from the viewpoint of energy consumption, it can be considered that resistance components of a power supply that outputs the voltage of the trapezoidal shape or the triangular shape are large compared with resistance components of the actuator and resistance components of the circuits. In other words, a place where energy is mainly consumed simply shifts from the actuator and the circuits to the power supply. A total heat value and total energy consumption do not change although a heat value of the actuator decreases.
In an ink jet system, it is important to reduce heat generation in the vicinity of sections that jet inks. If the power supply that outputs the voltage of the trapezoidal wave or the triangular wave is adopted and the place of energy consumption shifts from the actuator and the circuits to the power supply, even if the total heat value does not change, this seems to be valuable in terms of temperature management.
However, when the power supply that outputs the voltage of the trapezoidal shape or the triangular shape must be adopted, actually, the power supply is arranged near the actuator and the circuits, then, heat of the power supply is unexpectedly transmitted to an ink jet head. This is because, in general, it is difficult to accurately transmit a high-speed waveform having large power over a long distance. When it is attempted to transmit a high-speed arbitrary waveform without distortion, it is necessary to match impedances of a driving system, a line system, and a reception system. However, when the matching is performed, power is consumed by matching resistance.
As explained above, even from the viewpoint of heat generation near the ink jet head, it is also important to reduce total energy consumption. When the energy consumption is examined, it is important to consider a system including a power supply.
As explained above, when the charging and discharging operation of the actuator Z1 is performed once, the energy extracted from the power supply is “Q·E” and the heat generation of the entire actuator and circuits is also about “Q·E”. The energy “Q·E” is useless power that hardly changes to operation energy of the actuator Z1. The energy “Q·E” causes a problem of temperature rise and a problem such as an increase in size of an apparatus shape and an increase in cost involved in an increase in capacity of the power supply.
Energy actually converted into operation energy of the actuator Z1 in the energy supplied from the power supply is energy consumed by only a smaller part of the “resistance components of the actuator Z1”. For example, when an ink droplet “6 (pL)” having specific gravity “1” is jet at speed of “10 (m/sec)”, energy received by the ink droplet is represented by the following formula:(1/2)·6·1012 (kg)·10 (m/sec)2=0.3 (nj)
If charging and discharging of voltage 20 (V) is applied to the actuator Z1 having capacitance 500 (pF) once in order to jet the ink droplet, energy consumption is represented by the following formula:500·1012 (F)·20 (V)2=200 (nj)
Energy efficiency at this point is 0.15%. The remaining 99.85% of energy changes to heat.
The driving device shown in FIG. 1 drives the actuator Z1 with forward charging and backward charging. Specifically, after the actuator Z1 is charged forward performed by turning on the switch elements S12 and S21, when the actuator Z1 is charged by backward energization performed by turning on the switch elements S22 and S11, amplitude corresponding to “2·E”, twice as high as the output voltage “E” of the DC power supply P, can be obtained as the amplitude of the actuator Z1. When the actuator Z1 is charged backward performed by turning on the switch elements S22 and S11 first and the actuator Z1 is charged backward performed by turning on the switch elements S12 and S21 next, the amplitude corresponding to “2·E”, twice as high as the output voltage “E” of the DC power supply P, can also be obtained.
If there is no discharging period of a closed circuit formed by turning on the switch elements S12 and S22 as indicated by the broken line arrow in FIG. 1, and immediately backward charging at the start point of T2 in FIG. 2 without discharging, power consumption should be “4·Q·E”, when is as same as charging with twice voltage.
However, if a discharging path of a closed circuit is formed by turning on the switch elements S12 and S22 as indicated by the broken line arrow in FIG. 1 to discharge the voltage of the actuator Z1 to near zero (V) between charging forward and charging backward, the power consumption can be reduced. It is same if the sequence is opposite as forward charging after backward charging.
It is effective to set a discharging period in this case shorter than a quarter of a period of peculiar oscillation of the ink jet head and long enough for sufficiently reducing voltage across the actuator Z1.
If the discharging period is set between the forward charging and backward charging in this way, the power consumption is halved compared to backward charging without discharging period of charging with twice voltage. While in the former case, the energy lost is [C·(2·E)2]/2=·C·E2, but in the later case, the energy lost is [C·(2·E)2]/2=2·C·E2.
The effect and the principle are explained in detail in JP-A-2000-185400 (U.S. Pat. No. 6,504,701).
However, an amount of power consumption reduced with this technique is only up to a half of power consumed without discharging period.
In order to further reduce the power consumed with this technique, it is necessary to increase the number of power supplies and perform discharging in plural stages. A technique for increasing the number of power supplies and reducing power consumption is disclosed in JP-A-2005-288830, JP-A-2007-98795, and the like. However, in the technique, although power consumption can be reduced, a driving device is complicated. In particular, a large number of power supplies are necessary.
In JP-A-2008-23813, the inventor of the present invention analyzes the existing power consumption reducing measures and proposes a new power consumption reducing method for reducing power consumption using an inductor. On the other hand, the present invention is devised to obtain effects close to those of JP-A-2008-23813 using an idea different from JP-A-2008-23813 and with a configuration simpler than that disclosed in JP-A-2008-23813.
From a viewpoint different from heat generation and energy consumption, the driving device in the past has problems explained below.
In the discharging indicated by the broken line arrow in FIG. 1, the potential at a drain (which functions as a source during the discharging) of the switch element S12 is higher than the voltage “E” for an instance. The switch element S12 is often placed in an integrated circuit together with other elements. In order to secure reliability of the integrated circuit, the potential at a back gate (not shown) of the switch element S12 has to be set to highest potential in the integrated circuit. Therefore, in the integrated circuit required to have high reliability, usually, it is necessary to prepare another power supply having voltage higher than the voltage “E” and give the voltage of the power supply to the back gate (not shown) of the switch element S12. However, in this case, the number of power supplies increases, power supply voltage of a level higher than operating voltage of the actuator Z1 is necessary, and an integrated circuit having high withstanding voltage that can cope with the power supply voltage is necessary.
In other words, the operating voltage of a drivable actuator has to be set lower than upper limit voltage of an integrated circuit in use by peak voltage at the time of discharging. Because of this limitation, for example, an operating frequency of the actuator Z1 falls or driving force decreases and highly viscous inks cannot be jet.